[Users] 4th order finite differencing in Exact

[Erik Schnetter]

I have just committed the patch that implements 4th order finite
differencing in Exact. Thorn Exact uses finite differencing to
calculate the extrinsic curvature. This feature (which is disabled by
default) can lead to a much more accurate extrinsic curvature.

We can now think about enabling this by default; at the moment I
cannot see any ill effect from this.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/